1. Field of the Invention
The present invention relates to a process for the preparation of (co)polymers of .alpha.-olefins.
More specifically, the present invention relates to a process for the preparation of polyethylene or copolymers mainly based on ethylene.
2. Discussion of the Prior Art
It is known in literature that ethylene, and .alpha.-olefins in general, can be polymerized using soluble complexes of transition metals of group IV combined with an organic oxyderivative of aluminium, generally called aluminoxane.
For example, Die Makromoleculare Chemie Rapid Communications, 12, 663-667 (1991) specifies that the complex Zr(CH.sub.2 Ph).sub.4 is active in the polymerization of ethylene, whereas Die Makromoleculare Chemie Rapid Communications, 9(2), 51-55, (1988) indicates that the compound Ti(O--Bu).sub.4 is an efficient catalyst in the polymerization of propylene. These articles, however, do not specify the molecular weights of the polymers obtained nor the MWD (Molecular Weight Distribution).
The U.S. Pat. No. 5.079.205, on the other hand, describes aryloxidic derivatives of metals of group IVB, such as zirconium and hafnium, having general formula M(OR).sub.2 X.sub.2 or M(OR)X.sub.3, wherein M represents the metal, R a phenoxy group diversely substituted and X a halogen, as catalysts for the preparation of polyethylene having a high molecular weight (Mw of about 300,000-500,000).
The preparation of N,N'-dialkylcarbamates of transition metals of groups IVB and VB is described in Chemische Berichte, 120, 955-964, (1987) and Journal of the Chemical Society Dalton Transactions, 693-698, (1991). These complexes are obtained by the treatment of the chloride of the transition metal with carbon dioxide and an aliphatic amine by means of a simple and convenient synthesis process.